This invention relates to an amplifier device, more particularly a transistor amplifier device capable of eliminating a secondary distortion caused by the variation in the current amplification coefficient of the transistor at any load resistance due to the variation in the collector current.
A typical prior art amplifier device shown in FIG. 1 comprises a first stage constituted by a pair of field effect transistors and a second stage constituted by an emitter grounded transistor having a constant current load. As shown, the amplifier device comprises an input terminal 10 adapted to receive an input signal, an output terminal 11, a differential amplifier constituted by a pair of juntion type field effect transistors 12 and 13, a PNP transistor 14 acting as a class "A" amplifier, and NPN transistors 15 and 16. The transistor 16 constitutes a constant current drive circuit for the differnetial amplifier constituted by the field effect transistors 12 and 13. The drain electrode of the field effect transistor 12 is connected to the base electrode of the transistor 14 and to the positive pole +B of a voltage source through a resistor 18, and the source electrode is connected to the source electrode of the field effect transistor 13 and these source electrodes are connected to the collector electrode of the transistor 16 in common. The gate electrode of the field effect transistor 12 is connected to the input terminal 10. The drain electrode of the field effect transistor 13 is connected to the positive pole +B while the gate electrode is connected to the output terminal 11 via a resistor 20 and grounded by a resistor 19. The collector electrode of the transistor 14 is also connected to the output terminal 11 and the emitter electrode is connected to the positive pole +B through a resistor 21. The collector electrode of the transistor 15 is connected to the output terminal 11, the emitter electrode is connected to the negative pole -B of the voltage source via a resistor 22 and the base electrode is connected to the juncture between resistor 23 and a diode 25. The diode 25 is forwardly connected in series with a diode 26 between the negative pole -B and the resistor 23. The emitter electrode of the transistor 16 is connected to the negative pole -B via a resistor 28 and the base electrode is connected to the juncture between resistors 23 and 29, the other end of the resistor 29 being grounded. A resistor 30 is connected between the input terminal 10 and the ground.
An input signal applied to the input terminal 10 is amplified by the field effect transistor 12 and the output thereof is amplified by the transistor 14 of the second stage to produce an amplified output at the output terminal 11.
In the conventional amplifier device as shown in FIG. 1 since the first stage comprises a differential amplifier its secondary distortion is small. On the other hand, since the second stage transistor 14 constitutes a class A type amplifier its linearity directly affects distortion of the output.
FIG. 2 shows one example of the characteristics of the collector current I.sub.C with reference to the collector-emitter voltage V.sub.CE when the transistor 14 is operated. In FIG. 2, the ordinate represents the collector current I.sub.C and the abscissa the collector-emitter voltage V.sub.CE. The V.sub.CE - I.sub.C characteristic curves are depicted for base current I.sub.B varying with a step of 2 .mu. A and a load resistor R.sub.L = 65 Kohms.
Generally, the value of resistor 18 is sufficiently larger than the input impedance of the transistor 14 so that it can be considered that transistor 14 operates in response to a current input. Accordingly, the current amplification coefficient h.sub.fe of the transistor 14 varies according to the collector current I.sub.C so that a secondary distortion will appear at the output terminal. More particularly, in FIG. 2 when I.sub.C - .DELTA..sub.C .DELTA.I.sub.B along the load resistance line (65 Kohms) is considered a curve as shown in FIG. 3 is obtained. FIG. 3 shows the I.sub.C - .DELTA.I.sub.C /.DELTA.I.sub.B characteristic where the transistor 14 comprises a transistor, for example 2SA810. When the transistor 14 is current-driven by selecting its operating point at 1.5mA, a secondary distortion as shown in FIG. 4 would always result in which the phase relationship is the same as that in FIG. 1 and wherein (a) shows the fundamental wave form, (b) the output wave form and (c) the distortion. For the purpose of decreasing the second order distortion, the transistor 14 may be replaced by a cascade circuit, or a drive impedance having a suitable value may be selected for the transistor 14. The substitution of a cascade circuit can eliminate the variation in the current amplification coefficient h.sub.fe caused by the variation in the collector output resistance by imposing a constant voltage load on the transistor 14 and can prevent the decrease in the bandwidth due to the collector-base capacitance Cob. However, this solution complicates the circuit construction and is not economical. According to the latter solution, that is the selection of a suitable drive impedance is preferred, wherein the variation of V.sub.BE - I.sub.C characteristic has an inverse relationship in phase with respect to that of I.sub.B - .DELTA.I.sub.C /.DELTA.I.sub.B characteristic. It then is possible to decrease the distortion by the cancelling effect of the generated distortion where the drive impedance is suitably selected. However, this solution is not effective because the distortion caused by V.sub.BE - I.sub.C characteristic contains a large quantity of higher harmonics.